According to the knowledge made by the applicant, presently gas heating hot water heater occupies great market shares. The existing gas heating hot water heater is a kind of instantly available hot water heating equipment, and its basic structure contains two separated closed systems, one for heating by the user and the other for using hot water. Its defects are shown as: because this equipment can't store water, when the user plans to apply heating system and hot water system at the same time, temperature control of hot water supply system is difficult, especially when the temperature of incoming water is low and water consumption is great, the required hot water temperature can't be reached.
After search we found that the Chinese patent 200920126995 has represented a technical scheme combining gas hot water system with heating system, whose characteristics is to add a coil heat exchanger in water tank, to transfer the heat in water tank to heating route. Although this system structure is simple, heat exchange speed is slow because the heat exchange process from water tank to heating circulation is shown as water-water. For the users with high heating demand, heat exchange area must be very large; therefore volume of the water tank is unfit for the family users.